A fourth type of RTX determinant was identified in and was designated When expressed in , recombinant ApxlVA showed a weak haemolytic activity and cohaemolytic synergy with the sphingomyelinase (-toxin) of These activities required the presence of an additional gene, ORF1, that is located immediately upstream of . The gene product could not be detected in cultures grown under various conditions ; however, pigs experimentally infected with serotypes 1, 5 and 7 started to produce antibodies that reacted with recombinant ApxlVA 14 d post-infection, indicating that is expressed In addition, sera from pigs naturally and experimentally infected with any of the serotypes all reacted with recombinant ApxlVA. The gene from the serotype 1 type strain Shope 4074 encodes a protein with a predicted molecular mass of 202 kDa which has typical features of RTX proteins including hydrophobic domains in the N-terminal half and 24 glycine-rich nonapeptides in the C-terminal half that bind Ca. The glycine-rich nonapeptides are arranged in a modular structure and there is some variability in the number of modules in the ApxIVA proteins of different serotypes of The deduced amino acid sequences of the ApxlVA proteins have significant similarity with the iron-regulated RTX proteins FrpA and FrpC, and to a much lesser extent with other RTX proteins. The gene could be detected in all serotypes and seems to be species-specific. Although the precise role of this new RTX determinant in pathogenesis of porcine pleuropneumonia needs to be determined, is the first induced toxin gene that has been described in


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